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| <h1 id="drop-elaboration"><a class="header" href="#drop-elaboration">Drop elaboration</a></h1> |
| <h2 id="dynamic-drops"><a class="header" href="#dynamic-drops">Dynamic drops</a></h2> |
| <p>According to the <a href="https://doc.rust-lang.org/reference/destructors.html">reference</a>:</p> |
| <blockquote> |
| <p>When an initialized variable or temporary goes out of scope, its destructor |
| is run, or it is dropped. Assignment also runs the destructor of its |
| left-hand operand, if it’s initialized. If a variable has been partially |
| initialized, only its initialized fields are dropped.</p> |
| </blockquote> |
| <p>When building the MIR, the <code>Drop</code> and <code>DropAndReplace</code> terminators represent |
| places where drops may occur. However, in this phase, the presence of these |
| terminators does not guarantee that a destructor will run. That’s because the |
| target of a drop may be uninitialized (usually because it has been moved from) |
| before the terminator is reached. In general, we cannot know at compile-time whether a |
| variable is initialized.</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>let mut y = vec![]; |
| |
| { |
| let x = vec![1, 2, 3]; |
| if std::process::id() % 2 == 0 { |
| y = x; // conditionally move `x` into `y` |
| } |
| } // `x` goes out of scope here. Should it be dropped? |
| <span class="boring">}</span></code></pre> |
| <p>In these cases, we need to keep track of whether a variable is initialized |
| <em>dynamically</em>. The rules are laid out in detail in <a href="https://rust-lang.github.io/rfcs/0320-nonzeroing-dynamic-drop.html">RFC 320: Non-zeroing |
| dynamic drops</a>.</p> |
| <h2 id="drop-obligations"><a class="header" href="#drop-obligations">Drop obligations</a></h2> |
| <p>From the RFC:</p> |
| <blockquote> |
| <p>When a local variable becomes initialized, it establishes a set of “drop |
| obligations”: a set of structural paths (e.g. a local <code>a</code>, or a path to a |
| field <code>b.f.y</code>) that need to be dropped.</p> |
| <p>The drop obligations for a local variable x of struct-type <code>T</code> are computed |
| from analyzing the structure of <code>T</code>. If <code>T</code> itself implements <code>Drop</code>, then <code>x</code> is a |
| drop obligation. If <code>T</code> does not implement <code>Drop</code>, then the set of drop |
| obligations is the union of the drop obligations of the fields of <code>T</code>.</p> |
| </blockquote> |
| <p>When a structural path is moved from (and thus becomes uninitialized), any drop |
| obligations for that path or its descendants (<code>path.f</code>, <code>path.f.g.h</code>, etc.) are |
| released. Types with <code>Drop</code> implementations do not permit moves from individual |
| fields, so there is no need to track initializedness through them.</p> |
| <p>When a local variable goes out of scope (<code>Drop</code>), or when a structural path is |
| overwritten via assignment (<code>DropAndReplace</code>), we check for any drop |
| obligations for that variable or path. Unless that obligation has been |
| released by this point, its associated <code>Drop</code> implementation will be called. |
| For <code>enum</code> types, only fields corresponding to the “active” variant need to be |
| dropped. When processing drop obligations for such types, we first check the |
| discriminant to determine the active variant. All drop obligations for variants |
| besides the active one are ignored.</p> |
| <p>Here are a few interesting types to help illustrate these rules:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>struct NoDrop(u8); // No `Drop` impl. No fields with `Drop` impls. |
| |
| struct NeedsDrop(Vec<u8>); // No `Drop` impl but has fields with `Drop` impls. |
| |
| struct ThinVec(*const u8); // Custom `Drop` impl. Individual fields cannot be moved from. |
| |
| impl Drop for ThinVec { |
| fn drop(&mut self) { /* ... */ } |
| } |
| |
| enum MaybeDrop { |
| Yes(NeedsDrop), |
| No(NoDrop), |
| } |
| <span class="boring">}</span></code></pre> |
| <h2 id="drop-elaboration-1"><a class="header" href="#drop-elaboration-1">Drop elaboration</a></h2> |
| <p>One valid model for these rules is to keep a boolean flag (a “drop flag”) for |
| every structural path that is used at any point in the function. This flag is |
| set when its path is initialized and is cleared when the path is moved from. |
| When a <code>Drop</code> occurs, we check the flags for every obligation associated with |
| the target of the <code>Drop</code> and call the associated <code>Drop</code> impl for those that are |
| still applicable.</p> |
| <p>This process—transforming the newly built MIR with its imprecise <code>Drop</code> and |
| <code>DropAndReplace</code> terminators into one with drop flags—is known as drop |
| elaboration. When a MIR statement causes a variable to become initialized (or |
| uninitialized), drop elaboration inserts code that sets (or clears) the drop |
| flag for that variable. It wraps <code>Drop</code> terminators in conditionals that check |
| the newly inserted drop flags.</p> |
| <p>Drop elaboration also splits <code>DropAndReplace</code> terminators into a <code>Drop</code> of the |
| target and a write of the newly dropped place. This is somewhat unrelated to what |
| we’ve discussed above.</p> |
| <p>Once this is complete, <code>Drop</code> terminators in the MIR correspond to a call to |
| the “drop glue” or “drop shim” for the type of the dropped place. The drop |
| glue for a type calls the <code>Drop</code> impl for that type (if one exists), and then |
| recursively calls the drop glue for all fields of that type.</p> |
| <h2 id="drop-elaboration-in-rustc"><a class="header" href="#drop-elaboration-in-rustc">Drop elaboration in <code>rustc</code></a></h2> |
| <p>The approach described above is more expensive than necessary. One can imagine |
| a few optimizations:</p> |
| <ul> |
| <li>Only paths that are the target of a <code>Drop</code> (or have the target as a prefix) |
| need drop flags.</li> |
| <li>Some variables are known to be initialized (or uninitialized) when they are |
| dropped. These do not need drop flags.</li> |
| <li>If a set of paths are only dropped or moved from via a shared prefix, those |
| paths can share a single drop flag.</li> |
| </ul> |
| <p>A subset of these are implemented in <code>rustc</code>.</p> |
| <p>In the compiler, drop elaboration is split across several modules. The pass |
| itself is defined <a href="https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc_mir_transform/src/elaborate_drops.rs">here</a>, but the <a href="https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc_mir_transform/src/elaborate_drops.rs">main logic</a> is |
| defined elsewhere since it is also used to build <a href="https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc_mir_transform/src/shim.rs">drop shims</a>.</p> |
| <p>Drop elaboration designates each <code>Drop</code> in the newly built MIR as one of four |
| kinds:</p> |
| <ul> |
| <li><code>Static</code>, the target is always initialized.</li> |
| <li><code>Dead</code>, the target is always <strong>un</strong>initialized.</li> |
| <li><code>Conditional</code>, the target is either wholly initialized or wholly |
| uninitialized. It is not partly initialized.</li> |
| <li><code>Open</code>, the target may be partly initialized.</li> |
| </ul> |
| <p>For this, it uses a pair of dataflow analyses, <code>MaybeInitializedPlaces</code> and |
| <code>MaybeUninitializedPlaces</code>. If a place is in one but not the other, then the |
| initializedness of the target is known at compile-time (<code>Dead</code> or <code>Static</code>). |
| In this case, drop elaboration does not add a flag for the target. It simply |
| removes (<code>Dead</code>) or preserves (<code>Static</code>) the <code>Drop</code> terminator.</p> |
| <p>For <code>Conditional</code> drops, we know that the initializedness of the variable as a |
| whole is the same as the initializedness of its fields. Therefore, once we |
| generate a drop flag for the target of that drop, it’s safe to call the drop |
| glue for that target.</p> |
| <h3 id="open-drops"><a class="header" href="#open-drops"><code>Open</code> drops</a></h3> |
| <p><code>Open</code> drops are the most complex, since we need to break down a single <code>Drop</code> |
| terminator into several different ones, one for each field of the target whose |
| type has drop glue (<code>Ty::needs_drop</code>). We cannot call the drop glue for the |
| target itself because that requires all fields of the target to be initialized. |
| Remember, variables whose type has a custom <code>Drop</code> impl do not allow <code>Open</code> |
| drops because their fields cannot be moved from.</p> |
| <p>This is accomplished by recursively categorizing each field as <code>Dead</code>, |
| <code>Static</code>, <code>Conditional</code> or <code>Open</code>. Fields whose type does not have drop glue |
| are automatically <code>Dead</code> and need not be considered during the recursion. When |
| we reach a field whose kind is not <code>Open</code>, we handle it as we did above. If the |
| field is also <code>Open</code>, the recursion continues.</p> |
| <p>It’s worth noting how we handle <code>Open</code> drops of enums. Inside drop elaboration, |
| each variant of the enum is treated like a field, with the invariant that only |
| one of those “variant fields” can be initialized at any given time. In the |
| general case, we do not know which variant is the active one, so we will have |
| to call the drop glue for the enum (which checks the discriminant) or check the |
| discriminant ourselves as part of an elaborated <code>Open</code> drop. However, in |
| certain cases (within a <code>match</code> arm, for example) we do know which variant of |
| an enum is active. This information is encoded in the <code>MaybeInitializedPlaces</code> |
| and <code>MaybeUninitializedPlaces</code> dataflow analyses by marking all places |
| corresponding to inactive variants as uninitialized.</p> |
| <h3 id="cleanup-paths"><a class="header" href="#cleanup-paths">Cleanup paths</a></h3> |
| <p>TODO: Discuss drop elaboration and unwinding.</p> |
| <h2 id="aside-drop-elaboration-and-const-eval"><a class="header" href="#aside-drop-elaboration-and-const-eval">Aside: drop elaboration and const-eval</a></h2> |
| <p>In Rust, functions that are eligible for evaluation at compile-time must be |
| marked explicitly using the <code>const</code> keyword. This includes implementations of |
| the <code>Drop</code> trait, which may or may not be <code>const</code>. Code that is eligible for |
| compile-time evaluation may only call <code>const</code> functions, so any calls to |
| non-const <code>Drop</code> implementations in such code must be forbidden.</p> |
| <p>A call to a <code>Drop</code> impl is encoded as a <code>Drop</code> terminator in the MIR. However, |
| as we discussed above, a <code>Drop</code> terminator in newly built MIR does not |
| necessarily result in a call to <code>Drop::drop</code>. The drop target may be |
| uninitialized at that point. This means that checking for non-const <code>Drop</code>s on |
| the newly built MIR can result in spurious errors. Instead, we wait until after |
| drop elaboration runs, which eliminates <code>Dead</code> drops (ones where the target is |
| known to be uninitialized) to run these checks.</p> |
| |
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